1. Field of the Invention
The present invention relates to a measure weight or presence of material contained in packets or envelopes.
2. Description of Related Art
Scintillation detectors are used as weight measuring sensors but have drawbacks. The scintillation detector uses a material called a scintillator, which, when it absorbs a unit of radiation, becomes activated and will deactivate by giving off light in the ultraviolet range through the process of scintillation. This ultraviolet light is then detected by a quite sensitive light detector known as a photomultiplier tube. The photomultiplier tube is closely coupled to the scintillator material so that ultraviolet light will pass into it. A detection signal from a first photocathode within the photomultiplier tube is typically multiplied by a series of dynode stages which are successively more positive in charge than the preceding dynode stage. The multiplied signal is then taken as an output signal.
A disadvantage of the scintillation detector is that it also uses lethal high voltages in the 1,000 to 3,000 volt range. In addition, the photomultiplier tube characteristics change and drift with use and age and with fluctuations in the high voltage level. This type of a detector is also prone to breakage since it is an evacuated glass tube. Because of its size, weight and cost, this type of detector also cannot be used in many types of environments.
Semiconductor diode particle detectors are also known. They consist of a p-type layer, an n-type layer and an optional intrinsic layer in the middle. This diode is sensitive to light and nuclear and atomic radiation when the same strikes the diode. Detectors of this type have been used for high resolution nuclear spectroscopy. The signal generated by this type of detector consists of the collection of charge released when the radiation loses energy in the sensitive region of the detector. This charge is collected across the p-type and n-type materials and is amplified with a high performance charge sensitive amplifier.
Typically these types of detection devices have a very small output signal. The output signal is so small that noise generated by thermal effects or impurities and imperfections in the silicon material will overcome the charge signal. For this reason, such detectors are typically operated at quite low temperatures, usually through the use of a liquid nitrogen coolant, to arrive at adequate performance. While this type of detector could be used in laboratory research, it cannot be practically used in a production environment.